In a vehicle according to the prior art, a power transmission system is provided on the route from a prime mover to a wheel, and a hydraulic control unit, an electromagnetic type control system or the like are known as a mechanism for controlling the condition of the power transmission system. One example of the hydraulic control unit for vehicles is disclosed in Japanese Patent Laid-Open No. 2002-115755. The vehicle suggested by this Laid-Open has an engine, and the engine torque is transmitted to the wheel through a torque converter, an automatic transmission and a differential. The automatic transmission comprises a geared transmission capable of setting a plurality of forward gear stages, and a reverse gear stage. Also, there are provided a C1 clutch, which is applied in case the forward gear stage is selected, and a C2 clutch, which is applied in case the reverse gear stage is selected. The C1 and C2 clutches are applied by raising the operating oil pressure to be fed, and released by reducing the operating oil pressure.
Also, there is provided a hydraulic control unit for controlling the condition of operating oil fed to the C1 clutch, the C2 clutch, the torque converter and so on. The hydraulic control unit has an oil pump driven by the power of the engine. The operating oil discharged from the oil pump is fed to a manual valve through a primary regulator valve. Two oil passages diverge from the manual valve. One of those oil passages is connected with a hydraulic chamber of the C1 clutch, and the other one is connected with a hydraulic chamber of the C2 clutch. On the route from the manual valve to the hydraulic chamber of the C1 clutch, a first oil passage and a second oil passage are arranged in parallel. The first oil passage is equipped with a selector valve, and the second oil passage is equipped with an orifice. Also, there is provided a line-pressure control solenoid for controlling the primary regulator valve. Moreover, a clutch oil passage is arranged to connect the primary regulator valve and the manual valve, and an accumulator having a piston and a spring is connected with the clutch oil passage. Furthermore, an accumulator control solenoid is provided on the route from the clutch oil passage to the accumulator.
In the hydraulic control unit thus constructed, the oil pump is driven by the engine power, and the operating oil discharged from the oil pump is fed to the clutch oil passage through the primary regulator valve. Pressure of the operating oil in the clutch oil passage is controlled by the line-pressure control solenoid. In case any of shift positions such as, D position, 4th position, 3rd position, 2nd position and L position is selected, the operating oil in the clutch oil passage is fed to the hydraulic chamber of the C1 clutch through the manual valve. As a result of this, the C1 clutch is applied. At the same time, the operating oil is discharged from the hydraulic chamber of the C2 clutch so that the C2 clutch is released. In case the later-described “ECO-run control”, is not carried out, the selector valve is closed when the operating oil is fed to the hydraulic chamber of the C1 dutch. Here, the “ECO-run control” is a coined term to describe a control to run the vehicle “ECOnomically and ECOlogically”. Therefore, the operating oil is fed to the hydraulic chamber of the C1 clutch comparatively slowly through the second oil passage. In case R position is selected, on the contrary, the operating oil in the clutch oil passage is fed to the hydraulic chamber of the C2 clutch through the manual valve so that the C2 clutch is applied. At the same time, the operating oil is discharged from the hydraulic chamber of the C1 clutch. Consequently, the C1 clutch is released.
According to the vehicle suggested by Japanese Patent Laid-Open No. 2002-115755, the aforementioned “ECO-run control” can be carried out. For example, in case the vehicle is running in an urban area and the shift position is D position, the engine is stopped automatically when the vehicle is waiting at the traffic light and an automatic stop condition is satisfied. The automatic stop condition can be exemplified by the conditions such as the accelerator is OFF, the brake is ON, the vehicle speed is zero or the like. On the contrary, the engine is restarted in case the automatic stop condition is no longer satisfied. Thus, the “ECO-run control” is the control to stop and restart the engine automatically, according to satisfaction or dissatisfaction of the automatic stop condition. In case the “ECO-run control” is carried out, the engine is stopped automatically so that the oil pump is halted and the oil is not fed to the C1 clutch. For this reason, a response of the C1 clutch may be degraded in case the C1 clutch is applied again after the engine is restarted.
According to the Japanese Patent Laid-Open No. 2002-115755, therefore, a rapid boosting control is carried out in case of feeding the operating oil to the C1 clutch when the engine is restarted. That is, the operating oil is promptly fed to the hydraulic chamber of the C1 clutch through the first oil passage by opening the selector valve, so that an application of the C1 clutch is thereby facilitated. In addition to the rapid boosting control, the accumulator control solenoid is opened to feed the operating oil reserved in the accumulator to the hydraulic chamber of the C1 clutch through the clutch oil passage. This increases flow rate of the operating oil fed to the hydraulic chamber of the C1 clutch.
Meanwhile, according to the Japanese Patent Laid-Open No. 2002-115755, a spring constant dominates operating characteristics of the accumulator which facilitates to increases the feeding amount of the operating oil to the hydraulic chamber of the C1 clutch. From this point of view, it is still required to improve feeding efficiency of the operating oil to the hydraulic chamber of the C1 clutch.